HIGH-DOSE-RATE INDUCED TEMPERATURE ARTIFACTS - THERMOMETRY CONSIDERATIONS FOR SIMULTANEOUS INTERSTITIAL THERMORADIOTHERAPY

Purpose: The goal of the present study,vas to investigate the effect o f high dose-rate radiation on a flouroptic thermometry system commonly used during microwave hyperthermia. Methods and Materials: Measuremen ts were performed by placing the flouroptic thermometry sensors at dis tances of less than or equal to 1.5, 5, 10, and 15 mm from a remote af terloading high dose-rate (192)IR source in a water bath (at two diffe rent temperatures) and in a tissue equivalent radiation bolus medium. A simulated volumetric clinical setup using a radiation bolus medium w as performed with thermometry sensors placed at 1.5, 7.5, 8.4, and 10. 6 mm from a scanning high dose-rate source. Results: It was found that high dose-rate radiation caused thermometry artifacts greater than 1. 5 degrees C within 2 min for flouroptic thermometers placed 1.5 mm fro m a 5 Ci activity high dose-rate source. Simple calculations showed th at artifacts of this magnitude could not be due to any heating caused by the energy deposited by the high dose-rate source. The artifact dec ayed, but was still evident 24 h after the exposure. The effect strong ly depended on distance with a 0.7 degrees C artifactual increase in t emperature seen for the probe 5 mm from the high dose-rate source. Mor eover, experiments performed under conditions that represented a clini cal setup with a 7 Ci high dose-rate source showed that for exposure t imes of 10 s, at distances of 1.5 mm, significant artifacts(> 0.5 degr ees C) are produced. Conclusions: These findings indicate that high do se-rate-induced temperature artifacts should be taken into account in the quality assurance procedures for the treatment of patients with si multaneous interstitial thermoradiotherapy.